Reductant dosing systems are typically used to reduce NOx emissions in large machines where space and weight considerations are not a concern, such as, for example, in locomotives and stationary power generation applications. The reductant is stored in a tank located on the machine and, as the machine operates and produces exhaust, the reductant is pumped from the tank into the machine's exhaust system. The reductant reacts with exhaust at high temperatures to affect a selective catalytic reduction (SCR) of NOx within the exhaust.
A known shortcoming of dosing systems relates to the relatively high ambient temperatures at which some reductants freeze (about 12° F.). When the reductant freezes, it expands within the dosing system, causing damage to and/or clogging of intricate components such as injector nozzles. One way to inhibit freezing is to purge the system after use. To purge the dosing system, a purging fluid (typically air) is directed through system passages and components to push remaining reductant out of the system.
U.S. Patent Application Publication No. 2005/0252201 (the '201 publication, now abandoned) by Lecea et al. discloses an exemplary purging process. Specifically, the '201 publication discloses an urea storage tank that is connected to an exhaust system via a pump and a passage. Urea is injected into the exhaust system via an atomizer located on the passage. The '201 publication also discloses a purging system that purges reductant from the passage between the pump and the atomizer using compressed air. After compressed air is introduced to the passage, the mixture of air and reductant flows back into the tank or exits the system through the atomizer.